This invention relates to call disposition signal monitoring and more particularly, to an arrangement and method for the reliable detection of call progress tones transmitted through a telephone central office and indicative of the ultimate disposition of a live call switched therethrough. The achievement of reliable mechanized tone-detection is important in studying telephone network behavior characteristics for performance measurement, fault isolation and maintenance.
In the past, arrangements for detecting call progress and disposition tones in a live call environment in a tandem or toll switching center have been employed which bridged a tone detector across the tip and ring leads of the cross-office transmission path associated with the call to be monitored. Typically, a tone detector is attached to a circuit which has been seized by the incoming terminal path circuit, i.e., line or trunk circuit in order to establish the cross-office connection. For example, in a common control switching office the tone detector is attached to a sender after the latter has been seized for use by the incoming trunk circuit in the tandem or toll switching office. A tone detector thus bridged across the transmission path is able to detect and, if necessary, to record the received disposition signal tones which indicate the ultimate disposition of the call, e.g., busy, reorder, recorded voice announcement, called customer answer, etc. While the above-described arrangement is successful in detecting live call disposition signals, it has the disadvantage that the tone detector is exposed to several possible sources that may interfere with reliable detection and disposition categorization.
First, the calling party may generate random and spurious speech energy while the tone detector is bridged on and may thus interfere with proper and reliable signal tone detection. Secondly, the calling party, upon hearing a tone that signifies the unavailability of the called circuit, e.g., busy tone or reorder tone, may release the established connection before the monitoring equipment has sampled a sufficient number of periods of the disposition tones in order to make a correct determination thereof.
It would therefore be advantageous to provide an arrangement for reliably monitoring call disposition signals without exposing the tone detector to speech or noise interferences emanating from the originating source and without risking prematue termination by the calling party before a proper determination of the call disposition can be made.